This invention relates to a method and means for processing an input digital signal, and more particularly to a processing operation by which the signal is scrambled for transmission through a noisy communication channel.
U.S. Patent application Ser. No. 724,170 filed Sept. 17, 1976, in the names of Daniel Graupe et al, (which is hereby incorporated by reference), discloses a method of and means for scrambling an input speech signal that is to be transmitted through a communication channel, and for descrambling the received signal to obtain a representation of the input signal. In such application, an n-level digitizing of the input signal is performed at the sending end allowing transformation of the levels of the digitized signal to other levels using a preselected transformation code whereby the transformed signal is a scrambled version of the input signal. The transformed signal can be transmitted through a communication channel such as an acoustic medium, a telephone link, a CB radio link, etc. At the receiving end of the channel, an n-level digitization of the received signal is performed followed by an inverse transform of the levels of the digitized signal using the inverse of the preselected transformation code applied to the digitized input signal. The inversely transformed signal is then converted into an analog signal which is representative of the input signal.
Where the transmission channel contains significant noise, which is added to the transmitted signal during its transmission, the received signal may differ significantly from the transmitted signal with the result that the inverse transformation yields a representation of the input signal that is degraded in proportion to the amount of noise in the channel.
It is therefore an object of the present invention to provide a new and improved technique for processing an audio-frequency signal to permit scrambled transmission through a noisy communication channel, while allowing for recovery of a reasonable representation of the original audio-frequency signal.